The present invention relates to a constant current circuit formed to include field effect transistors in a compound semiconductor integrated circuit.
In order to realize constant current sources, a current mirror circuits have typically been utilized in the past. For instance, an example thereof wherein Si bipolar elements are used is shown at page 234 of "Analysis and Design of Analog Integrated Circuits," by Gray Meyer, Second Edition, John Wiley & Sons Inc., 1984, and an example thereof wherein Si MOS field effect transistors are used is shown at page 710 of the same literature.
FIG. 2 shows the case where the current mirror circuit is constructed of field effect transistors (hereinafter abbreviated as FETs). In FIG. 2, F21 and F22 denote FETs, R21 a linear resistance element as a load, and V.sub.1 a power source. I.sub.ref denotes a reference current flowing through R21 and F21, and the circuit is so constructed that an output constant current I flows through F22 with said reference current used as an input current.
This constant current circuit of FIG. 2 has a defect in that the gate-source voltage V.sub.gs of two FETs F21 and F22 increases and also the output current I increases when the input reference current I.sub.ref increases beyond a prescribed value owing to the voltage fluctuation of the power source V.sub.1, the nonuniformity in the resistance R.sub.21 and the distribution of the threshold voltage of FET F21.
FIG. 3 shows a constant current circuit which was disclosed by Japanese Patent Publication No. 16463/1971. In this circuit, a linear resistance R.sub.51 set as in the following is connected between the collector and the base of a bipolar transistor Q.sub.l on the input side: EQU (qR.sub. 51/kT)=1
where q indicates an amount of charge of electrons, k is a Boltzmann's constant, and T - absolute temperature.
Accordingly, an increase in the base-emitter voltage V.sub.BE of the transistor Q.sub.l caused by an increase in the input reference current I.sub.ref is canceled by an increase in a voltage drop R.sub.51 I.sub.ref of the linear resistance R.sub.51, and consequently an output current I flowing through the collector of a bipolar transistor Q.sub.2 on the output side remains at a substantially fixed value.